Optik 122 (2011) 1872–1875 Contents lists available at ScienceDirect Optik j o ur nal homepage: www.elsevier.de/ijleo Theoretical modeling of a nonlinear asymmetric metal-clad planar waveguide based sensors Dinesh Kumar, Vivek Singh ∗ Department of Physics, Banaras Hindu University, Varanasi (U.P.), India a r t i c l e i n f o Article history: Received 14 August 2010 Accepted 9 December 2010 Keywords: Nonlinear waveguide Metal clad waveguide Asymmetric planar waveguide Reflectivity Sensitivity a b s t r a c t The present paper deals with the theoretical modeling of a nonlinear asymmetrical metal-clad planar waveguide for sensor application to achieve high sensitivity. We take the appropriate field distributions for the proposed structure and impose the boundary conditions to derive the mode equation and other necessary formulae for the proposed sensor. The effect of nonlinearity on the sensitivity of the waveguide is studied. Also, we have compared our computed results with the results given for a similar sensor having five layers structure. It is observed that the sensitivity of the sensor is improved by introducing a nonlinear material in the cover medium. © 2011 Elsevier GmbH. All rights reserved. 1. Introduction It is well known that optical waveguide based sensor is a self contained integrated device which can provide specific and quantitative analytical information about the presence or the con- centration of biological/chemical molecules. Significant attempts have been made towards the development of evanescent field sen- sors for biological and chemical sensing which are based on the interaction of evanescent field of a planar waveguide mode with the surrounding media and the resulting perturbation of waveguide mode effective index [1–3]. Varieties of optical sensors based on evanescent wave sensing technology have been proposed and com- mercialized [4–6]. The working principle of this waveguide sensor is to measure the change of effective refractive index due to change in the cover refractive index medium. Since this type of sensors has large numbers of applications [7–11] therefore it is necessary to increase the sensitivity of the sensors. Recently metal clad opti- cal waveguide sensors have been used to increase sensitivity and detect biological objects of micrometer size [12–14]. Very recently Taya et al. have been shown that the sensitivity of an optical waveg- uide sensor can be dramatically enhanced by using a metamaterial with negative permittivity and permeability [15]. However, there has been increasing interest in the experimen- tal and theoretical studies of nonlinear effects on the propagation of electromagnetic waves in optical systems, because the trans- ∗ Corresponding author at: Department of Physics, Faculty of Science, Banaras Hindu University, Varanasi 221005 (U.P.), India. Tel.: +91 0542 2311685. E-mail address: viveks@bhu.ac.in (V. Singh). mission of these optical systems is a multi-valued function of input intensity [16–18]. Kaplan [19] found theoretically that input beam is an infinite plane wave, an interface between a linear and a non linear medium should have a reflection coefficient which exhibit hysteresis as a function of light intensity. An exact solu- tion of Maxwell’s equations, which describes the propagation of p-polarized nonlinear surface wave propagating in layered struc- ture have been studied by Mihalache and Fedyanin [20,21] and they have found the bistable state of p-polarized nonlinear sur- face waves provides that the power flow in the wave is the control parameter. Taya et al. also propose a nonlinear three layer slab waveguide optical sensor and they found that in the case of homo- geneous sensing the sensitivity of the effective refractive index to variations of the cladding index can be improved [22]. Despite these attempts much more work needs to be done before optical waveguide based-sensors become commercially viable option for sensing. In this paper we analyze a metal-clad planar waveguide based sensor having five layers structure as shown in Fig. 1. For the com- paring purpose all the parameters of waveguide are chosen as the same as those used by Singh and Kumar [14]. Here we propose to take 1 mm thick glass plate as a substrate [n S = 1.48534(FK51A)], a metal cladding of 30 nm silver (n M = 0.065 + i4.0) a 500 nm thick polystyrene (PS) film (n F = 1.59); the affinity layer is approximately 30–60 nm thick and coated with carbon source or hydrophobic plastic. The necessary formula has been derived and given in this paper to study the performance of proposed biosensor under dif- ferent conditions related to its constituents. The present study is organized in the following way. Section 2 is devoted to the derivation of mode equation and other necessary formula for the 0030-4026/$ – see front matter © 2011 Elsevier GmbH. All rights reserved. doi:10.1016/j.ijleo.2010.12.031